This disclosure relates to fuel cells and polymer electrolyte membranes used in fuel cells.
FIG. 1 is a diagram illustrating a typical polymer electrolyte fuel cell. In FIG. 1, a membrane electrode assembly (MEA) 150 is supported by gasket 120 and sandwiched between a fuel delivery system 130 and an oxidizer delivery system 140. The MEA 150 includes a polymer electrolyte membrane (PEM) 155, an anode catalyst layer 153 on an anode surface of the PEM 155, an anode diffusion layer 151 covering the anode catalyst layer 153, a cathode catalyst layer 157 on a cathode surface of the PEM 155, and a cathode diffusion layer 159 covering the cathode catalyst layer 157.
The PEM is a proton-permeable, electrically non-conductive membrane that allows transport of protons through the PEM from the anode to the cathode while preventing electrons from passing through the PEM. An example of a PEM typically used in fuel cells is a perfluorosulfonic acid membrane such as sulfonated tetrafluorethylene copolymer membranes available as Nafion® plastic membrane from E.I. Dupont de Nemours and Company of Wilmington, Del. The anode catalyst layer 153 includes a catalyst such as platinum for increasing the anode reaction rate. The anode diffusion layer 151 is typically a porous electrical conductor such as carbon paper or cloth that conducts electrons generated by the anode reaction from the anode catalyst layer 153 to an external load while allowing transport of anode reaction reactants and products between the anode catalyst layer 153 and fuel delivery system 120. The cathode catalyst layer 157 includes a catalyst such as platinum for increasing the cathode reaction rate. The cathode diffusion layer 159 is typically a porous electrical conductor such as carbon paper or cloth that conducts electrons from the external load to the cathode catalyst layer 157 while allowing transport of cathode reaction reactants and products between the cathode catalyst layer 157 and the oxidizer delivery system 140.
Fuel delivery system 130 delivers fuel to the anode catalyst layer 153 and removes reaction products, if any, from the anode. Fuel may be delivered to the anode catalyst layer 153 through a fuel plenum, not shown, or through fuel distribution channels 135. Fuel delivery system 130 may include a flow distributor that distributes the fuel evenly over the anode side of the MEA, a humidifier to control water content at the anode, and valves and pumps to control the flow of materials into and out of the anode. Typically, the humidifier, and pumps are housed external to the fuel cell but contribute to the overall portability of the fuel cell. In a hydrogen fuel cell, where the fuel is hydrogen, no reaction products, other than protons, are produced at the anode side. When the fuel is methanol, the methanol can either be converted to hydrogen using a reformer or can be applied directly to the anode. When methanol is fed directly to the anode, the fuel cell is called a direct methanol fuel cell (DMFC) and water must be supplied with the methanol to the anode catalyst layer at the anode. If sufficient water is not provided at the anode, the methanol may be incompletely oxidized to form reaction products such as formaldehyde or formic acid. The incomplete oxidation of the fuel reduces the energy generated by the fuel cell and decreases the efficiency of the fuel cell.
Oxidizer delivery system 140 delivers oxidizer via oxidizer delivery channel 145 to the cathode catalyst layer 157 and removes reaction products from the cathode. Oxidizer is generally oxygen and may be conveniently provided as air although pure oxygen or enriched air may be used as the oxidizer. Protons transported through the PEM from the anode recombine with the oxidizer at the cathode to produce water as a cathode reaction product. The water produced at the cathode may be supplied to the fuel delivery system in a DMFC. Oxidizer delivery system 140 may include a flow distributor that distributes the oxidizer over a cathode surface of the MEA, a humidifier, a water reservoir, and pumps, blowers, and valves to control the material flows to and from the cathode. Typically, the humidifier, reservoir, pumps, and valves are housed external to the fuel cell but contribute to the overall bulk of the fuel cell.